Residual gases originating from a sulphur plant in which sulphur is produced by the process of controlled oxidation of sour gas containing H.sub.2 S, known as the Claus process, contain of the order of 0.2 to 2% by volume of sulphur compounds, a high proportion of which consists of H.sub.2 S, the remainder consisting of SO.sub.2, COS, CS.sub.2 and sulphur in vapour and/or vesicular form.
Such residual gases are commonly treated to reduce the overall content of sulphur compounds as much as possible with the aim Of allowing them to be discharged to the atmosphere, after they have been burnt, while conforming to the standards imposed by legislation relating to atmospheric pollution and simultaneously of recovering these sulphur compounds in a form contributing to increasing the yield of upgradable products produced from the sour gas treated in the sulphur plant.
Various processes for performing the treatment of a Claus sulphur plant residual gas are known, and especially processes comprising a combined treatment of hydrogenation and hydrolysis of the residual gas with a view to converting the sulphur compounds which it contains solely into the form of H.sub.2 S, followed by a cooling of the effluent resulting from the said combined treatment to temperature which is appropriate for condensing most of the water vapour present in this effluent and, finally, a treatment of the gaseous effluent depleted in water vapour to remove the H.sub.2 S therefrom, it being possible for this removal of H.sub.2 S to be carried out either by absorption of H.sub.2 S by means of a selective solvent which can be regenerated or else by controlled catalytic oxidation of H.sub.2 S.
Among the processes of the abovementioned type, comprising a removal of H.sub.2 S by catalytic oxidation to sulphur, there features a process in which, after the combined treatment of hydrogenation and hydrolysis of the Claus residual gas and cooling of the resulting gaseous effluent in order to condense most of the water in it, the effluent depleted in water vapour is passed with a controlled quantity of a gas containing free oxygen, at an appropriate temperature, in contact with a catalyst for H.sub.2 S oxidation to form a gas stream containing H.sub.2 S and SO.sub.2 in a H.sub.2 S:SO.sub.2 molar ratio substantially equal to 2:1 together with elemental sulphur in vapour phase, and the said gas stream is then brought, after cooling below 160.degree. C. and optional separation from the sulphur which it contains, into contact with a Claus catalyst operating at a sufficiently low temperature for the sulphur formed by reaction of H.sub.2 S with SO.sub.2 to be retained on the catalyst with production of a residual gas effluent with a greatly reduced content of sulphur compounds which is subjected to burning before discharge to the atmosphere, and the sulphur-laden Claus catalyst is purged at regular intervals with the aid of a nonoxidizing gas at a temperature of between 200.degree. C. and 500.degree. C. to vaporize the sulphur retained by this catalyst and thus to ensure the regeneration of the latter, and the regenerated catalyst is then cooled to the temperature required for being again brought into contact with the gas containing H.sub.2 S and SO.sub.2, that is to say with the gas stream produced by the oxidation.
In such a process the cooling of the gaseous effluent resulting from the combined treatment of hydrogenation and hydrolysis applied to the residual gas is conducted so as to bring the water vapour content of the cooled gaseous effluent to a value lower than approximately 10% by volume, and this results in the production of a large quantity of residual water contaminated with H.sub.2 S, which must be treated to remove H.sub.2 S therefrom. In addition, although the control of the quantity of gas containing free oxygen, employed in the stage of catalytic oxidation of H.sub.2 S in Claus stoichiometry, may be efficient, it remains possible for unconverted oxygen to be entrained in the gas stream containing H.sub.2 S and SO.sub.2 produced by the catalytic oxidation, which is brought into contact with the Claus catalyst, the result being a risk of deactivation of the said Claus catalyst. Finally, since the ultimate stage of removal of the sulphur compounds is the Claus reaction, the quality of the purification carried out is therefore dependent on the quality of the control of the flow of air injected into the oxidation reactor.